The present invention relates to a focusing screen for use in a single-reflex lens camera, a camera with an electronic shutter, a TV camera, or other types of cameras.
In taking pictures with a camera, the operator focuses the camera while looking at the image formed on a focusing screen via an optical system including imaging lenses. The focusing screen, which has a large number of light diffusing asperities on its surface, is conventionally made of ground glass or has many small lenses arranged in a regular or cyclic pattern. In the latter type of focusing screens, the surfaces of the small lenses are made spherical or in the shape of a right circular cone.
FIG. 16A is a plan view showing part of a prior art focusing screen having small spherical lenses, FIG. 16B is a partial cross section taken along a line IIIB--IIIB in FIG. 16A, and FIG. 16C is a partial cross section taken along a line IIIC--IIIC in FIG. 16A.
In FIGS. 16B and 16C, the focusing screen is indicated by 10, with individual small spherical lenses denoted by 12. In the example shown, small spherical lenses 12 are formed on the surface of the focusing screen 10, and as shown in FIG. 16A, a large number of small lenses 12 are disposed in a regular pattern to form a honeycomb structure.
FIG. 17a is a plan view showing part of a prior art focusing screen having small lenses of right circular conical shape, FIG. 17B is a partial cross section taken along a line IVB--IVB in FIG. 17A, and FIG. 17C is a partial cross section taken along a line IVC--IVC in FIG. 17A.
In FIGS. 17B and 17C, the focusing screen is indicated by 14, with individual conical lenses denoted by 16. In the embodiment shown, small lenses 16 of right circular conical shape are formed on the surface of the focusing screen 14, and as shown in FIG. 17A, a large number of small lenses 16 are disposed in a regular pattern to form a honeycomb structure.
A problem with such a focusing screen is that because of the presence of many asperities, shadows are prone to occur in some parts of the image formed on the screen and such shadows, being disagreeable to the viewer of the screen (i.e., the operator of the camera), sometime create an unpleasant sensation. These shadows become increasingly conspicuous as the imaging lens is stopped down to a narrow diaphragm opening.
In a conventional focusing screen made of ground glass (this type of focusing screen is referred to hereinafter simply as the former focusing screen), small asperities of different sizes are formed in a random pattern, as a result of which the shadows are more conspicuous than in a prior art focusing screen provided with a regular pattern of small lenses (this type of focusing screen is referred to hereinafter simply as the latter focusing screen). Although the intensity of the shadows produced in the latter focusing screen provided with a regular pattern of small lenses is reduced to a less conspicuous level than in the former focusing screen, an unclear image is still formed on the screen, and it has been difficult to reduce the disagreeable sensation to a level that is tolerable for practical purposes.
FIGS. 18 and 19 are spectrum characteristic diagrams of two prior art focusing screens provided with small spherical and conical lenses, respectively. Angles in degrees are plotted on the x and y axes of each diagram. The characteristic diagrams shown in FIGS. 18 and 19 were constructed by computer simulation of the spectral intensity light diffused by each focusing screen when a defocused image of a point source of light (.lambda.=550 .mu.m) is formed on the screen. The spectral intensity of light is shown to be proportional to the diameter of circles plotted in each diagram. The refractive indices of the optical materials of which the small lenses and the focusing screen are made are assumed to be equal to each other.
FIG. 18 shows the results of calculation with the pitch P and radius r assumed to be 20 .mu.m and 34.083 .mu.m, respectively. If the center of the sphere including the surface of a small spherical lens 12 and the radius of this sphere are written as Q and r, respectively, the pitch P denotes the distance between adjacent centers Q (see FIGS. 16A to 16C).
As is clear from FIG. 18, if small spherical lenses 12 are provided on a focusing screen, the spectral intensity of the first-order light is relatively low, whereas the intensity of the zero-order light is somewhat high, producing great variations in the spectral intensity of light, particularly in the third and lower orders of light.
FIG. 19 shows the results of calculation with the pitch P and angle .THETA. being assumed to be 20 .mu.m and 10 degrees, respectively. If the axis of the right circular cone including the surface of a conical small lens 16 is written as S, the pitch P denotes the distance between adjacent axes S, and the angle .THETA. signifies the angle formed between line V normal to axis S and the sloping side of the right circular cone (see FIGS. 17A-17C).
As is clear from FIG. 19, if small lenses in the shape of a right circular cone are provided on a focusing screen, the spectral intensity of the first-order light is relatively low, again producing great variations in the spectral intensity of light, particularly in the third-and lower orders of light.
On the basis of these results of computer simulation, the present inventor surmised that the disagreeable sensation produced by unclear images formed on the prior art focusing screens are chiefly caused by the great variations in the spectral intensity of light.